Case Study: Neutrino Observatories

The Super-Kamiokande neutrino observatory uses 50,000 tons of pure water surrounded by 11,200 sensitive light detectors 1 kilometer below ground in Japan. Neutrinos from space interact with the water and produce flashes of blue light. Technicians on a raft check the photodetectors (far right). Photo courtesy of the ICRR (Institute of Cosmic Ray Research), The University of Tokyo.

The image of an astronomer peering through the eyepiece of an enormous telescope in a mountaintop dome late at night is still a stereotype in popular culture. While this sort of thing was, in fact, common a generation ago, astronomers today rarely “look” through telescopes. Instead they look at computer monitors while directing the operation of sensitive electronic light detectors attached to telescopes. The astronomer does not even have to be anywhere near the telescope, which could, for example, be in space.

A wide range of modern detectors can “see” not only visible light from celestial objects but also every other kind of light in the electromagnetic spectrum, from radio waves to gamma rays, all invisible to human eyes. However, light is not the only carrier of information across space.

Subatomic particles called neutrinos permeate the universe. In fact, during the next second, trillions of neutrinos from cosmic sources will pass through your body with no effect. Like visible light from stars, radio waves from galaxies, and X-rays from matter spiraling down black holes, neutrinos can also reveal something of the cosmos.

Neutrinos entered theoretical physics in 1930 as a way to understand beta decay, the process by which a radioactive atomic nucleus spits out an electron. Experiments showed that the total energy of the nucleus plus the ejected electron after the decay was less than the energy of the initial nucleus. This appeared to violate the conservation of energy, a fundamental principle of physics which says that energy is neither created nor destroyed.

The Austrian physicist Wolfgang Pauli, so convinced of the conservation principle, made a daring intellectual leap by proposing that an unknown particle carries off the missing energy. To agree with the observations, this particle had to be electrically neutral, possess practically zero mass, and move at the speed of light. Pauli suggested this in a letter to his colleagues but didn’t publish it. He understood that a scientific theory is almost worthless unless it can be tested by observation or experiment, and he was concerned that such particles could never be detected. Then in 1932, James Chadwick discovered the neutron, a particle with nearly the same mass as the proton but no electric charge. Pauli gained confidence and published his idea. The physicist Enrico Fermi named Pauli’s particle the neutrino, meaning “little neutral one” in Italian.

In 1987, the most spectacular supernova seen in four centuries appeared in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. The exploding star released a burst of neutrinos observed on Earth. This composite image made in 1994–97 shows rings of gas expanding from the dying star. Hubble Space Telescope. Photo courtesy of AURA/STScI/NASA.

The hypothetical neutrino served to explain beta decay, and even led Fermi to recognize that there must be a new nuclear force, now called the weak force, if neutrinos actually exist. But for years these ideas remained unconfirmed. Other physicists would poke fun at Pauli, some even calling the neutrino “the little neutral one who isn’t there.”

The task remained to detect these things, and that would be very difficult. Neutrinos have almost no interaction with matter. In theory, a high-energy neutrino could pass through a hundred light-years of solid steel without hindrance. And radioactive atoms, the only source then known for the hypothetical neutrinos, should emit relatively few of them. The picture changed in the 1950s, however, with the advent of nuclear reactors. Such power plants would produce a flood of neutrinos if Pauli and Fermi were correct.

Clyde Cowan and Frederick Reines decided to use a nuclear reactor to search for neutrinos. They designed a 10-ton detector to record the tiny spark of light expected from the rare interaction of a passing neutrino with a proton inside the device. They placed their detector next to the newly-completed Savannah River nuclear power plant in South Carolina. Day after day, they recorded data and slowly gathered evidence from the occasional flashes of light. After a few years, Cowan and Reines had enough data to prove that neutrinos actually exist. Pauli was naturally ecstatic and made sure his colleagues knew about the result.

By this time, physicists realized that the nuclear reactions that power the stars must also produce neutrinos in enormous numbers. Raymond Davis decided to try observing these cosmic neutrinos. But in order to do so, he would have to shield his detector from cosmic rays, the high-energy charged particles constantly bombarding the Earth from space. His solution was to place the detector deep underground. Cosmic rays cannot penetrate hundreds of meters of rock, but for neutrinos the entire Earth would be transparent. In 1967, Davis installed a large tank of cleaning fluid in a deep gold mine in South Dakota. Any neutrino passing through this tank had a tiny probability of hitting a chlorine atom in the cleaning fluid and converting it to a radioactive argon atom, which could easily be detected. This experiment found about one neutrino every few days. They were attributed to the Sun.

Neutrino observatories around the world have continued to collect solar neutrinos. According to astrophysical theory, the Sun should emit about two percent of its energy in the form of neutrinos. But the number of neutrinos actually detected on Earth is only about a third as many as predicted by the theory. Perhaps there is something we don’t understand about the Sun or about neutrinos. The problem of the“missing solar neutrinos” continues to occupy the attention of many investigators.

In 1987, neutrino astronomy made a stunning and unexpected advance. The two most sensitive neutrino observatories in the world, one in Japan and the other in Ohio, had recently become operational when the first naked eye supernova since 1604 blazed forth in the sky. That event, called SN1987A, announced the explosive death of a massive star, which radiated as much visible light during one day as the entire Milky Way Galaxy. Such an explosion ought to produce a strong burst of neutrinos. The supernova was in the Large Magellanic Cloud, a companion galaxy to our Milky Way. It was just close enough for the enormous dose of neutrinos passing through the Earth to trigger a response in the two neutrino observatories.

Both observatories detected a 12-second burst of neutrinos about three hours before the supernova became optically visible. The explosion in the core of the star produced the neutrinos which raced through the overlying stellar mass at the speed of light and out into space. Three hours later the shock wave from the core reached the surface of the star and blew it apart to make the visible supernova explosion. No one could doubt that the observed burst of neutrinos came from SN1987A. The number of neutrinos detected by these two observatories allowed physicists to estimate how much matter was involved in nuclear reactions during the supernova explosion.

This event was a watershed in neutrino experiments and has prompted the construction of more sensitive detectors, all placed deep below the Earth’s surface. These mineshaft observatories, a far cry from mountaintop optical telescopes, should reveal more about the nature of supernovas and other high energy cosmic events, as well as provide more clues about the elusive neutrinos.